1. Field of the Invention
This invention relates to a video signal recording and/or reproduction apparatus in which video signals are digitized, then stored in a memory, and then recorded, and the digitized video signals are reproduced through another memory for storage of the video data.
2. Description of the Related Art
In the field of art of recording and/or reproduction apparatus, for example, video tape recorders, there has been proposed a technique of digitizing the video signals by PCM (pulse code modulation) before they are recorded on the recording medium and retrieving them from the digital form when in reproduction. For the purpose of sampling the video signals (the number of quantized bits: 8), the frequency is taken usually at a value equal to 4 fsc's (frequency of color subcarrier). And, either when recording or when reproducing, the digitized video signals, or the video data, are written in a field emory such as dynamic RAM in synchronism with a clock, and then read out therefrom in synchronism with another clock. An example of combination of the frequencies of the write- and read-clocks is shown in Table 1 below.
TABLE 1 ______________________________________ Write-Clock Read-Clock ______________________________________ When recording 4 fsc's 63 KHz When reproducing 63 KHz 4 fsc's ______________________________________
In Table 1, the value of 63 KHz is obtained by dividing the frequency of the master clock which has a value of 11.58 MHz and is used to form the PCM signal for the audible range. Also, the value of 4 fsc's is obtained from the master clock of the VTR (video tape recorder) or the synchronizing signal separated out of the television signal inputted when in recording.
In the conventional video signal recording and/or reproduction apparatus, however, for the aforesaid video data to be written in the aforesaid field memory or to be read out therefrom, no synchronism is taken between the write-clock and the read-clock. In the case that the frequencies of the write- and read-clocks differ from each other as has been described above, therefore, there is a possibility of occurrence of an appreciable phase discrepancy. This led to a problem that the writing and reading operations for the field memory overlap each other. In more detail, on assumption that, as shown in FIG. 1, the writing of the video data is performed when the write-clock rises, and the reading of the video data is performed when the read-clock falls, because these clocks are not in synchronism, it will then eventually occur that the writing time and the reading time overlap each other. This gives rise to problems such as that of failing in normally performing the writing or the reading.